There are two types of high pressure fuel pumping systems for gas turbine engines. The first type utilizes a positive displacement pump (typically a gear pump). The other type utilizes a centrifugal pump. The fuel metering units for these types of fuel systems are substantially different in design, application and practice due to the fact that positive displacement pumps provide a predetermined flow rate based on pump speed (a flow generation source), whereas a centrifugal system generates pressure (a pressure generation source) proportional to pump speed squared.
Examples of positive displacement pump fuel metering systems are disclosed in U.S. Pat. No. 4,458,713 to Wernberg, U.S. Pat. No. 5,433,237 to Kao et al., and U.S. Pat. No. 6,381,946 to Wernberg et al. In these systems, the speed of the pump determines the fuel flow supplied to the fuel metering unit. For positive displacement systems, it is necessary for the fuel metering unit to recirculate (e.g. bypass and return) a portion of the pumped fuel flow back to the inlet of the high pressure pump. This is due to the fact that the pump is sized large enough to provide enough fuel flow to meet the maximum demanded fuel flow rates for the gas turbine engine.
Centrifugal pumps, by contrast do not provide a predetermined flow rate based upon speed. The fuel metering unit for centrifugal pumping systems throttles (restricts) pump flow rather than bypasses flow.
Referring to a prior art centrifugal system schematically shown in FIG. 1, which generally depicts the relevant portions of a typical centrifugal pump type engine fuel system, the engine fuel system includes a fuel tank and a low pressure centrifugal boost pump. The boost pump supplies fuel to a variable displacement starting pump and to two high speed centrifugal pumps, one for the core engine and the other for the afterburner. The fuel for the high speed centrifugal pump for the core engine is controlled with a fuel metering valve that is positioned by an electrohydraulic servovalve (EHSV), which is turn in controlled by the FADEC (full authority digital electronic controller). A position sensor (such as a LVDT or linear variable displacement transducer) provides metering valve position feedback to the FADEC. A throttle valve is arranged in series with the metering valve. The throttle valve provides a variable restriction orifice in the fuel flow path that controls the pressure drop across the fuel metering valve (at 50 PSI for example). The throttle valve opens and closes the variable restriction orifice to maintain the pressure drop constant. To keep the metering valve pressure drop constant with excellent accuracy as is typically desired, the system of FIG. 1 employs a pressure sensor which typically contains a bellows or diaphragm that senses pressure drop across the fuel metering valve. Typically, this pressure sensor positions a low friction, low flow first stage valve which in turn positions the larger throttle valve. This is mathematically an integrating type system as flow from the first stage valve is integrated by the second stage throttle valve piston until the error in the predetermined pressure drop is zero.
Unfortunately, incorporating the plumbing, multiple stages, valves and sensors to provide accurate control over metering valve pressure drop accuracy such as that schematically illustrated in FIG. 1 has added substantial weight, size, and expense. It has also reduced dynamic performance, stability and the reliability of centrifugal pump metering systems. These are all disadvantages, particularly in aircraft applications where there is always a constant desire to reduce weight while maintaining or increasing performance and reliability.